Noupe.com has posted a top ten list of what they consider some of the best PHP/Ajax-based webmail clients (as based on the user interface and ease of use).
Employees need to access their email from wherever they happen to be '" on the road, at customer sites, remote offices, and at home. [...] Today i wanted to share with you 10 AJAX-based webmail client that delivers the look and feel, usability and performance of a desktop application. Some of the webmail clients listed below are open source code that can be easily modified to better suit user's needs, while others are not. So You must check their license to know the rules.
Included in their list is software like:
Some of these tools offer more than just email too - they can include things like collaboration tools and mobile versions right out of the box.